Method and apparatus for synchronizing alternating quadratures differential binary phase shift keying modulation and demodulation arrangements

ABSTRACT

Methods, apparatus, and systems for alternating quadratures differential binary phase shift keying (AQDBPSK) modulation and demodulation radically increase overall energy efficiency of a communication system due to the combined effect of efficient utilization of transmitter power, high noise immunity of modulation and demodulation, fast synchronization, and simple implementation. Efficient utilization of transmitter power is achieved because radical reduction of side lobe regeneration provided by AQDBPSK allows operation of all amplifiers of the transmitter analog signal path, including power amplifiers, in energy efficient nonlinear mode (for example, they can operate as class C amplifiers) without causing interchannel interference. AQDBPSK radically reduces side lobe regeneration by limiting phase transitions between adjacent in time modulator output symbols to ±90° and smoothly executing the transitions. High noise immunity of AQDBPSK is achieved due to transmitting modulator input data by binary 0° and 180° phase differences between adjacent in time same parity modulator output symbols and optimal noncoherent demodulation of the received same parity symbols. Use of noncoherent demodulation reduces synchronization time, and fast synchronization substantially lowers energy consumption during burst transmissions. Proposed methods of modulation, noncoherent demodulation, and synchronization can easily be realized using the simplest digital signal processing operations and the simplest digital circuits. These methods can also be implemented in a universal reconfigurable digital modem. This enables significant reduction of power consumption in real world applications. In addition, entirely digital realization of the proposed methods, apparatus, and systems make them resistant to all destabilizing factors and simplifies their practical implementation; and fast and reliable synchronization increases time allocated for data transmission when transmission time is limited.

CROSS REFERENCE TO RELATED APPLICATION

Application of Yefim Poberezhskiy, entitled ALTERNATING QUADRATURESDIFFERENTIAL BINARY PHASE SHIFT KEYING MODULATION AND DEMODULATIONMETHOD, filed on even date herewith.

Application of Yefim Poberezhskiy, entitled APPARATUS FOR PERFORMINGALTERNATING QUADRATURES DIFFERENTIAL BINARY PHASE SHIFT KEYINGMODULATION AND DEMODULATION, filed on even date herewith.

TECHNICAL FIELD

This disclosure relates to communication systems, methods, andapparatus. More particularly, this disclosure relates to modulation anddemodulation techniques used in communication systems that involvereduced side lobe regeneration and efficient utilization of transmitterpower. This disclosure also relates to synchronization schemes for suchcommunications systems, methods, and apparatus.

BACKGROUND

In many communication systems, for example, in systems using frequencyhopping, time division multiple access (TDMA), or fast transmissions ofshort messages, data are transmitted in bursts. In these and similarcases, noncoherent demodulation is preferable because it allows radicalreduction of synchronization time and complexity. Reduction ofsynchronization time and complexity increases overall energy efficiencyof a communication system. Noncoherent demodulation is also advantageouswhen simple system implementations are required.

High overall energy efficiency in a communication system is alwaysdesirable, and it is absolutely necessary in systems usingbattery-powered radios. The overall energy efficiency is determined byseveral factors. The first factor is the amount of time it takes for thetransmitter and the receiver to be synchronized. Shorter synchronizationtimes mean more time can be spent for the transmission of data. Thisfactor is especially important for communication systems using bursttransmissions. The second factor is the level of circuit complexityneeded to implement the chosen modulation/demodulation method. Simplecircuit design contributes to overall energy efficiency because itreduces power consumption of a transmitter and especially a receiver.This factor is exceptionally important for short-distance communicationswhere the power of a transmitted signal is comparable with or even lowerthan power consumption of the receiver. The third factor is noiseimmunity of a modulation/demodulation method used in the system. Highnoise immunity reduces the energy necessary for transmission of givenamount of information with required reliability. The fourth factor isefficiency of transmitter power utilization provided by themodulation/demodulation method. High efficiency of transmitter powerutilization lowers the power consumption required for given transmittedpower. Other factors that influence overall energy efficiency of acommunication system are not related to the modulation/demodulationmethods. Therefore, they are not considered here.

Thus, there is a great need for a modulation/demodulation technique thatis immune to noise and interference, efficiently uses transmitter power,and is simple to implement. Taking into account that noncoherentdemodulation reduces synchronization time and complexity, such a methodwill maximize overall energy efficiency of a communication systembecause it will improve all four factors that determine this efficiency.

Differential binary phase shift keying (DBPSK) is the most noise immunebinary modulation in additive white Gaussian noise (AWGN) channels thatallows noncoherent demodulation [1-4]. It also allows the simplestrealization of the modulators and noncoherent demodulators. Comparisonof DBPSK with M-ary modulation techniques that allow noncoherentdemodulation shows the following. When noncoherent demodulation is used,DBPSK is much more noise immune than any M-ary phase modulationincluding non-coherently demodulated differential quadrature phase shiftkeying (DQPSK) and differential offset quadrature phase shift keying(DOQPSK), although it has lower bandwidth efficiency. Non-coherentlydemodulated DBPSK is more bandwidth efficient than M-ary orthogonalkeying, and the difference in bandwidth efficiency between DBPSK andM-ary orthogonal keying increases with the growth of M. At the sametime, non-coherently demodulated DBPSK is more noise immune than M-aryorthogonal keying when M≦4. Only when M>4, does M-ary orthogonal keyingprovide higher noise immunity than non-coherently demodulated DBPSK.DBPSK allows the simplest practical realization compared to allalternative modulation techniques. It also provides the greatest easeand the highest noise immunity of bit synchronization and tracking.Additional advantage of DBPSK is the fact that the difference in noiseimmunity between noncoherently demodulated DBPSK and coherentlydemodulated BPSK is insignificant. Thus, the high noise immunity ofnoncoherent demodulation and synchronization as well as simplicity ofrealization and implementation are advantages of DBPSK.

The main drawback of DBPSK is inefficient utilization of the transmitterpower because of frequency side-lobe regeneration. Indeed, the passageof a filtered conventional DBPSK signal through nonlinear circuitscauses significant regeneration of the side-lobes of the signalspectrum. These side lobes can interfere with neighboring channels.

The cause for the side-lobe regeneration is as follows. A bandpasssignal modulated by conventional DBPSK has a constant envelope, and itsspectrum has significant side lobes. Filtering that suppresses theseside lobes radically changes the signal envelope. After this filtering,the envelope starts to go up and down, and 180° phase transitions, whichare inherent in conventional DBPSK and BPSK, drive the envelope to zero.Hard-limiting the signal, for example, in a class C amplifier, restoresits almost constant envelope at the expense of regeneration of thesignal spectrum side lobes and reduction of the noise immunity of thesignal reception.

To avoid side-lobe regeneration, all units of the transmitter analogsignal path including the transmitter power amplifier (PA) have to beoperated in linear mode (for example, class A). However, a linear modeof the transmitter analog signal path (and, first of all, a linear modeof its PA) does not allow efficient utilization of the transmitter powerdue to a high peak factor of filtered DBPSK and BPSK signals. As aresult, the transmitter power cannot be efficiently utilized.

Thus, despite its high noise immunity and simplicity DBPSK cannotprovide sufficiently high overall energy efficiency in a communicationsystem due to its inefficient utilization of the transmitter power.

It has to be stated that, although 180° phase transitions betweenadjacent symbols cause the side-lobe regeneration, the transmission ofdata by binary signals with opposite phases (antipodal signals) is thereason for high noise immunity of conventional BPSK and DBPSK. Reductionof the phase shift between signals from 180° to 90° radically lowersnoise immunity of the modulation, on the one hand, and fundamentallyreduces the side-lobe regeneration enabling better utilization oftransmitter power, on the other. These competing phenomena do not allowachieving sufficiently high overall energy efficiency of a communicationsystem with conventional BPSK and DBPSK.

Therefore, known methods [5-8] that mitigate regeneration of the signalspectrum side lobes in nonlinear amplifiers are based on providingsmooth phase transition from 0° to 180° and vice versa. These methodshave two significant drawbacks: (1) they do not allow noise immunenoncoherent reception of DBPSK (noncoherent reception of BPSK isimpossible), and (2) they cannot be implemented using only digitalcircuits (in other words, analog and/or mixed signal circuits arerequired for their implementation).

These methods are not energy efficient for two reasons. First, coherentdemodulation requires a much longer synchronization time thannoncoherent demodulation. Second, the impossibility of their digitalrealization increases power consumption and makes unfeasible theirimplementation as a part of a universal reconfigurable digital modemthat enables significant reduction of power consumption in real worldapplications.

SUMMARY

The problems outlined above are solved through a method, an apparatus,and a system involving the application of DBPSK principles tonon-adjacent in time bits in an incoming bit stream. The method,apparatus, and system enable radical increases in overall energyefficiency in communication systems.

Described herein are methods, apparatus, and systems for alternatingquadratures DBPSK (AQDBPSK) modulation and demodulation that radicallyincrease overall energy efficiency of a communication system due to thecombined effect of efficient utilization of transmitter power, highnoise immunity of modulation and demodulation, reduced synchronizationtime, and simple implementation.

Efficient utilization of transmitter power is achieved by eliminatingthe cause of side-lobe regeneration in nonlinear analog signal paths.Specifically, 180° phase transitions between adjacent symbols areexcluded. Phase transitions between adjacent symbols are limited to±90°, and these transitions are performed smoothly. The high noiseimmunity of AQDBPSK is provided by transmitting modulator input data bybinary 0° and 180° phase differences between adjacent in time sameparity the modulator output symbols and optimal noncoherent demodulationof the received same parity symbols.

Radical reduction of side lobe regeneration allows operation of allamplifiers of the transmitter analog signal path, including poweramplifier, in energy efficient nonlinear modes (for example, they canoperate as class C amplifiers) without causing interchannelinterference.

Noncoherent demodulation significantly reduces synchronization time,thus, lowering the energy required for synchronization and increasingtime allocated for data transmission.

Minimum power consumption of the modem and simplicity of itsimplementation are achieved because the proposed methods of modulation,noncoherent demodulation, and synchronization allow optimal realizationusing the simplest digital signal processing operations and digitalcircuits. These methods can easily be implemented in a universalreconfigurable digital modem. This enables significant reduction ofpower consumption in real world applications.

Thus, the main advantage of the methods, apparatus, and systemsaccording to the invention is enabling very high overall energyefficiency of communication systems. High overall energy efficiency of acommunication system is advantageous in all applications, and it isabsolutely necessary when battery powered radios (manpack radios,handheld radios, sensor radios, etc) are used in the system. The gain inoverall energy efficiency provided by the invention compared to knownmethods, apparatus, and systems depends on the roll-off factor of pulseshaping low-pass filters (LPFs) at the modulator output: the lower theroll-off factor is, the higher gain that can be achieved. In mostsituations, this gain is about 6 dB or more.

Fast and reliable synchronization is desirable in any communicationsystem, and it is especially important when data are transmitted inbursts. When transmission time is limited, fast and reliablesynchronization radically increases time allocated for data transmissionin addition to lowering the energy required for synchronization. The useof noncoherent demodulation instead of coherent demodulation allows atleast 4-fold reduction of synchronization time when matched filters areused for synchronization.

Illustrative objects of the invention are methods, apparatus, andsystems for alternating quadratures DBPSK (AQDBPSK) modulation anddemodulation that have one or more of the following distinctivefeatures.

-   -   1. Independent differential binary phase modulation of odd and        even data symbols, which is achieved (a) by differential        polarity modulation of the 2-bit symbols, corresponding to the        same parity input data bits, performed separately in I and Q        channels through multiplying the input symbols of a differential        modulator by its output symbols delayed by the time equal to the        length of two modulator input bits when different parity        modulator input data bits are sent to different quadrature        channels (the 1^(st) embodiment) or (b) by differential encoding        of same parity modulator input data bits by modulo 2 summing        differential encoder input bits with the encoder output bits        delayed by a time equal to the length of two modulator input        data bits lengths when all differentially encoded bits are sent        to both I and Q channels (the 2^(nd) and 3^(rd) embodiments).    -   2. Generation of a synchronization (sync) sequence by        alternating two pseudorandom, or pseudonoise (PN) sequences of        the same length (number of bits), the same starting time, and        the same bit rate, which is two times lower than the input data        bit rate.    -   3. Alternating of the PN sequences by selection of the first        half of each bit of the 1^(st) PN sequence and the second half        of each bit of the 2^(nd) PN sequence or vice versa and        combining them (a) after passing digital lowpass filters and        converting to intermediate frequency in the in-phase (I) and        quadrature (Q) channels when different parity bits are sent to        different quadrature channels (the 1^(st) embodiment) or (b)        just after the selection of the appropriate halves of bits when        the real valued sync sequence is sent to both I and Q channels        (the 2^(nd) and 3^(rd) embodiments).    -   4. Selection of the appropriate halves of the PN sequences' bits        provided by two switches, one of which operates under control of        the direct output of a generator of a sequence of alternating        ones and zeros (SAOZ) and another under control of the inverse        output of this generator. The bit rate at each output of the        SAOZ generator is equal to the modulator input data bit rate.    -   5. Forming ±90° phase shift between even and odd transmitter        output bandpass symbols (a) by sending all 2-bit symbols        corresponding to odd bits of the sync sequence and the modulator        input data to the I channel and all 2-bit symbols corresponding        to even bits of the sync sequence and the modulator input data        to the Q channel or vice versa and then by the separate        differential polarity modulation of 2-bit symbols in each        channel (the 1^(st) embodiment), or (b) by sending all bits of        the real-valued sync sequence and the same parity differentially        encoded data bits to both I and Q channels and then by modulo 2        summing these bits in one of the channels with the SAOZ bits        generated with the same bit rate as the modulator input data        bits (the 2^(nd) embodiment), or (c) by sending all 2-bit        symbols corresponding to the bits of the real-valued sync        sequence and the same parity differentially encoded data bits to        both I and Q channels and then by multiplying these two-bit        symbols in one of the channels by 2-bit symbols corresponding to        the SAOZ bits generated with the same bit rate as the input data        (the 3^(rd) embodiment).    -   6. Demodulation of received complex-valued baseband AQDBPSK        signals by separately integrating the signals in I and Q        channels during the time equal to the length of the modulator        input data bit and multiplying a current output signal of each        integrator by its output signal delayed by the time equal to the        length of two modulator input bits, summing the products, and        making a hard decision about a received symbol by utilizing only        the sign of the sum or a soft decision about the received symbol        by utilizing both sign and value of the sum.    -   7. Identifying the sync sequence and synchronizing a demodulator        of alternating quadratures differential binary phase shift        keying signals by sending a received sync sequence to a        complex-valued matched filter, which contains in each of its        quadrature channels (I and Q channels) a first real-valued        filter matched to the sequence consisting of the selected in the        modulator halves of the symbols corresponding to the 1^(st) PN        sequence and spaces between the adjacent in time halves and a        second real-valued filter matched to the sequence consisting of        the selected in the modulator halves of the symbols        corresponding to the 2^(nd) PN sequence and spaces between the        adjacent in time halves, and selecting the peak of the squared        absolute value of the signal at the complex-valued matched        filter output.

The modulation proposed in the invention is called alternatingquadratures differential binary phase shift keying (AQDBPSK) becauseeach transmitted symbol is in quadrature with the previous symbol (i.e.phase shift between all adjacent transmitted symbols can be equal onlyto ±90°), and modulator input data are transmitted by binary 0° and 180°phase differences between same parity modulator output symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified block diagram of an example of a communicationsystem in which the invention is useful.

FIG. 2 shows the structure of an example of a burst transmission handledby the communication system of FIG. 1.

FIG. 3 is a block diagram of a first embodiment of the inventioninvolving a sync sequence generator and modulator of FIG. 1 havingseparate format conversion and differential polarity modulation of sameparity 2-bit symbols in I and Q channels.

FIGS. 4 a-4 h are timing diagrams illustrating operation of the firstembodiment of the invention shown in FIG. 3 when operating in asynchronization mode.

FIG. 5 shows the signal constellation at the output of the apparatus ofFIG. 3.

FIGS. 6 a-6 h are timing diagrams illustrating the operation of thefirst embodiment of the invention shown in FIG. 3 when operating in adata transmission mode.

FIG. 7 is a block diagram of a second embodiment of the inventioninvolving a sync sequence generator and modulator of FIG. 1 having jointdifferential encoding of same parity input data bits for the I and Qchannels and separate format conversions in these channels.

FIGS. 8 a-8 j are timing diagrams illustrating operation of the secondembodiment of the invention shown in FIG. 7 operating in asynchronization mode.

FIG. 9 shows the signal constellation at the output of the apparatus ofFIG. 7.

FIGS. 10 a-10 f are timing diagrams illustrating the operation of thesecond embodiment of the invention shown in FIG. 7 when operating in adata transmission mode.

FIG. 11 is a block diagram of a third embodiment of the inventioninvolving a sync sequence generator and modulator of FIG. 1 having jointdifferential encoding of same parity input data bits and joint formatconversion for I and Q channels.

FIGS. 12 a-12 j are timing diagrams illustrating operation of the thirdembodiment of the invention shown in FIG. 11 operating in asynchronization mode.

FIGS. 13 a-13 f are timing diagrams illustrating the operation of thethird embodiment of the invention shown in FIG. 11 when operating in adata transmission mode.

FIG. 14 shows an embodiment of a synchronizer and demodulator inaccordance with this invention.

FIG. 15 shows an embodiment of a complex-valued matched filter withdirect form of realization of real-valued matched FIR filters in thesynchronizer and demodulator in accordance with this invention.

DETAILED DESCRIPTION

A simplified block diagram of a communication system is shown in FIG. 1.The communication system comprises a transmitter TX and receiver RX.Input data from line 10 enter the transmitter TX where they areprocessed and converted into a radio frequency (RF) signal radiated by atransmitting antenna 12. In the transmitter TX, the uncoded data fromline 10 are first processed by a transmit part 18 of a modem, comprisinga forward error correction (FEC) encoder 20 in series with a syncsequence generator and modulator 22. Then, the output signal of thetransmit part 18 of a modem is processed by a transmit signal path 24where the RF signal is formed and amplified, before being radiated bythe transmitting antenna 12. The RF signal radiated from thetransmitting antenna 12 is received by a receiving antenna 14 in thereceiver RX shown in FIG. 1. The receiver RX processes the signalreceived by antenna 14 and finally produces demodulated and decodedoutput data that are sent to line 16. The output data correspond to theinput data on line 10. The signal received by antenna 14 is processed ina receive signal path 26 and a receive part 28 of a modem comprising asynchronizer and demodulator 30 and an FEC decoder 32, before it is sentto the line 16.

When bandpass analog signal reconstruction is performed in thetransmitter TX, the TX signal path 24 includes digital circuits thatform a bandpass signal; a digital-to-analog converter (D/A); analogblocks that perform frequency conversion, filtering, preliminaryamplification, and level control of analog signals; and a poweramplifier (PA). The RX signal path 26 of the receiver RX with bandpasssampling includes preselector; analog blocks that perform frequencyconversion, amplification, filtering, and gain control;analog-to-digital converter (A/D); and a digital former of in-phase (I)and quadrature (Q) components. Frequency synthesizers, referenceoscillators, and some auxiliary blocks are not shown in FIG. 1.

The transmitter TX sends burst transmissions to the receiver RX. Eachburst transmission comprises two components, a synchronization sequenceand data. During a synchronization mode, generation and pulse shapingfiltering of a complex-valued baseband digital sync sequence isperformed in the sync sequence generator and modulator 22 (the TX part18 of the modem). In the TX signal path 24, this complex-valued basebanddigital sync sequence is converted into a real-valued bandpass digitalsignal, translated into the analog domain, transferred to a requiredradio frequency (RF), filtered, amplified, and finally transmitted. Inthe RX signal path 26, the sum of a received real-valued bandpass analogsignal corresponding to the sync sequence, noise, and interference isamplified, filtered, converted to an intermediate frequency (IF),translated into the digital domain, and digitally converted into acomplex-valued baseband digital signal. This complex-valued basebanddigital signal is used in the synchronizer and demodulator 30 (the RXpart of the modem) for bit, word, and frame synchronization of thecommunication system.

During the data transmission mode, the FEC encoder 20 in the TX part 18of the modem performs forward error correction (FEC) encoding of datathat contains header, message, and auxiliary information. The bits fromthe output of the FEC encoder 20 are input data of the sync sequencegenerator and modulator 22. For conciseness, these data are called“modulator input data” in the description below. After modulation in thesync sequence generator and modulator 22, a complex-valued basebanddigital signal that represents the encoded and modulated data enters theTX signal path 24. Signal processing within both TX and RX signal paths24 and 26 is the same for the synchronization and data transmissionmodes. In the RX part 28 of the modem, a complex-valued baseband digitalsignal is demodulated in the synchronizer and demodulator 30 and thendecoded in the FEC decoder 32. The synchronizer and demodulator 30 alsoincludes tracking system that maintains time alignment between incomingsignal and internally generated clock pulses.

The structure of a burst transmission comprising a sync sequencefollowed by encoded and modulated input data is shown in FIG. 2.

This invention can be used with virtually any method ofencoding/decoding and with any architecture of TX and RX signal paths.The architectures of the TX signal path with bandpass reconstruction andRX signal path with bandpass sampling are briefly described above onlybecause they usually allow achieving the best performance with any typesof modulation and encoding.

There are three preferable embodiments of the modulation and syncsequence generation in the invention. Since all the embodiments sendidentical signals to the receiver input, the same demodulation methodcan be used in the RX part of modem for all of these embodiments.

During a synchronization mode, a sync sequence is generated byalternating two pseudonoise (PN) sequences in all the embodiments. ThePN sequences have the same length (number of bits), the same startingtime, and the same bit rate equal to 0.5R, where R is the modulatorinput data bit rate. No data enters the sync sequence generator andmodulator 22 in this mode.

During the data transmission mode, modulator input data enters the syncsequence generator and modulator 22 with bit rate R. Consequently, thelength of each modulator input data bit is equal to T_(b)=1/R.

The 1^(st) Embodiment of Sync Sequence Generation and Modulation

The distinctive features of this embodiment of sync sequence generationand modulation compared to other embodiments are separate formatconversion and differential polarity modulation of the same parity 2-bitsymbols in in-phase I and quadrature Q channels.

FIG. 3 is a block diagram that illustrates the structure and operationof the sync sequence generator and modulator 22 of this embodiment.Besides the sync sequence generator and modulator 22, FIG. 3 shows theFEC encoder 20 and an input block 23 of the TX signal path 24. They areshown to clarify the succession of signal processing in the transmitterTX. At the same time, as mentioned above, the proposed AQDBPSKmodulation can be combined with any method of encoding/decoding and withany architecture of the TX signal path 24. In the sync sequencegenerator and modulator 22, switches S1, S2, S5, S6, S7, and S8,operated by a control unit 25 provide transition from thesynchronization mode to the data transmission mode and vice versa.

During the synchronization mode, 1^(st) and 2^(nd) PN sequences aregenerated by a first PN sequence generator 33 and a second PN sequencegenerator 34. These sequences can be linear or nonlinear, uniform orconcatenated depending on the application. In all cases, they shouldhave good autocorrelation and other pseudo-noise properties. Asmentioned above, the sequences should have the same length (number ofbits), and their generation should start simultaneously. Their bit rateshould be 0.5R, i.e. it should be half the modulator input data bit rateR.

The timing diagrams illustrating operation of the 1^(st) embodiment inthe synchronization mode are shown in FIGS. 4 a-4 h. The first 11 bitsof the 1^(st) and 2^(nd) PN sequences are depicted in FIG. 4 a and FIG.4 e, respectively. The bits of the 1^(st) PN sequence are marked byrhombuses, while the bits of the 2^(nd) PN sequence are marked bycircles. The length of each bit of both PN sequences is equal to 2T_(b).

The 1^(st) and 2^(nd) PN sequences, generated throughout thesynchronization mode, enter 1^(st) and 2^(nd) format converters 35 and36 through switches S1 and S2, respectively. In these format converters35 and 36, input bits (ones and zeros) are mapped into two-bit symbolsaccording to the following rule: 1→−1 and 0→1. The 2-bit symbols at theoutputs of the 1^(st) and 2^(nd) format converters 35 and 36corresponding to the first 11 bits of the 1^(st) and 2^(nd) PN sequencesare shown in FIGS. 4 b and 4 f, respectively.

2-bit symbols from the output of the 1^(st) format converter 35 enterswitch S3 controlled by signals from the direct output O of thegenerator 37 of a sequence of alternating ones and zeros (SAOZ). TheSAOZ generator 37 is a very simple unit. It can be realized using anyfrequency divider that forms an output square wave with levels 1 and 0.The timing diagram of the signal at the output O is shown in FIG. 4 c.As a result of the switch S3 operation, only the first half of each2-bit symbol passes through switch S3 (see FIG. 4 d). After passingthrough switches S5 and S7, the first halves of 2-bit symbols of the1^(st) PN sequence enter the pulse-shaping lowpass filter (LPF) 38 ofthe in-phase (I) channel 39. The filter 38 suppresses the side lobes ofthe signal spectrum, minimizes intersymbol interference, and performsinterpolation that increases the sampling frequency f_(s).

2-bit symbols from the output of the 2^(nd) format converter 36 enterswitch S4 controlled by signals from the inverse output O of the SAOZgenerator 37. The timing diagram of the signal at the output O is shownin FIG. 4 g. Therefore, only the second half of each 2-bit symbol canpass through switch S4 (see FIG. 4 h). After passing through switches S6and S8, the second halves of 2-bit symbols of the 2^(nd) PN sequenceenter the pulse-shaping LPF 40 of the quadrature (Q) channel 41. Thisfilter carries out the same functions as the LPF 38 in the I channel 39.

During the synchronization mode, the 1^(st) and 2^(nd) differentialmodulators 42 and 43 do not operate and are disconnected from otherunits by switches S5, S6, S7, and S8 (see FIG. 3).

Thus, the 1^(st) and 2^(nd) PN sequences at the input of the LPFs 38 and40 are I and Q components of the complex-valued baseband sync sequence.They are represented by 2-bit symbols of length T_(b) corresponding tothe bits of the PN sequences and spaces (see FIGS. 4 d and 4 h). 2-bitsymbols corresponding to the bits of the PN sequences can be equal only1 or −1, and all 2-bit symbols corresponding to the spaces are equal to0. Each 2-bit symbol corresponding to a space in the I channel 39coincides with a 2-bit symbol corresponding to a bit of the PN sequencein the Q channel 41 and vice versa.

As a result of processing in the LPFs 38 and 40, each 2-bit symbol istransformed into several multibit samples. The transformation allowssignificant smoothing of the shape of the modulator output signals. Dueto this smoothing, the tails of the symbols cover the spaces. However,the peak of each symbol at the output of the LPF 38 in the I channel 39approximately coincides with the middle of a space at the output of theLPF 40 in the Q channel 41 and vice versa, because the LPFs 38 and 40are finite impulse response (FIR) filters.

The outputs of the LPFs 38 and 40 in the I and Q channels 39 and 41 arethe outputs of the sync sequence generator and modulator 22. Thus, LPFs38 and 40 perform digital interpolation and pulse shaping of themodulator output signals. The entire signal processing is digital in thesync sequence generator and modulator 22. The digital baseband symbolsfrom the output of the LPFs 38 and 40 constitute a complex-valuedbaseband digital sync sequence.

As shown in FIG. 3, this complex-valued baseband digital sync sequenceenters the TX signal path 24. As stated above, any architecture of TXsignal path 24 can be used in the present invention. The input block 23of the TX signal path 24 shown in FIG. 3 forms an output signal to besent to a D/A converter that is a real-valued bandpass digital signal.Such a block 23 is related to architectures with bandpass analog signalreconstruction.

In block 23, the output signal of the LPF 38 in the I channel 39 ismultiplied by cos 2λf₀t in multiplier 44, and the output signal of theLPF 40 in the Q channel 41 is multiplied by sin 2πf₀t, in multiplier 45.Usually, f₀=0.25f_(s). Such relations between f₀ and f_(s) simplify themultiplications in multipliers 44 and 45. These multiplications are apart of frequency conversion of the baseband signals from the outputs ofthe LPFs 38 and 40 to an intermediate frequency f₀, and simultaneouslythey provide 90° phase shift between the signals in the I and Q channels39 and 41. The output signals of the multipliers 44 and 45 are added toeach other by adder 46 thus forming a real-valued bandpass digitalsignal corresponding to the sync sequence constructed of two alternatingPN sequences. This real-valued bandpass digital signal enters a digitalto analog converter D/A, and, after translation into the analog domainand further analog processing in the TX signal path 24, is transmittedby the antenna 12.

The odd symbols of this signal correspond to the bits of the 1^(st) PNsequence processed in the I channel 39, and the even symbols of thissignal correspond to the bits of the 2^(nd) PN sequence processed in theQ channel 41. Since the phase shift between the I and Q signalcomponents is equal to 90°, only +90° or −90° phase transitions betweenadjacent symbols are possible. This is illustrated by the signalconstellation corresponding to the 1^(st) embodiment of sync sequencegeneration and modulation shown in FIG. 5. In this signal constellation,black circles correspond to odd symbols transmitted over the I channel39, and white circles correspond to even symbols transmitted over the Qchannel 41. Transitions between circles with the same color areimpossible because all odd symbols come from the I channel 39, and alleven symbols come from Q channel 41.

Thus, only +90° or −90° phase transitions between adjacent symbols ofthe bandpass signal corresponding to the sync sequence are possible.These ±90° transitions are gradual because the symbols are smoothed inthe LPFs 38 and 40 of the I and Q channels 39 and 41. Such a signal cantolerate hard limiting without any significant regeneration of thespectrum side lobes. Therefore, all the amplifiers in the TX signal path24 including the PA can operate in nonlinear mode (for example, as classC amplifiers) that allows efficient utilization of transmitter power.

During the data transmission mode, input data from line 10 enter the FECencoder 20. The encoded data from the output of the FEC encoder 20(modulator input data) enter the sync sequence generator and modulator22 with bit rate R, and the 1^(st) and 2^(nd) PN sequences are notgenerated. The timing diagrams illustrating operation of the 1^(st)embodiment in the data transmission mode are shown in FIG. 6. The inputdata passes through switches S1 and S2 and enters the 1^(st) and 2^(nd)format converters 35 and 36 (see FIG. 6 a). In the format converters 35and 36, the modulator input data bits (ones and zeros) are mapped intotwo-bit symbols according to the rule: 1→−1 and 0→1 as shown in FIG. 6b. Each two-bit symbol has the same length T_(b) as the modulator inputdata bits.

Switches S3 and S4 under the control of the SAOZ generator 37 extractodd parity symbols from symbol stream at the output of the formatconverter 35 and even parity symbols from the symbol stream at theoutput of the format converter 36. The odd parity symbols are sent tothe I channel 39 and the even parity symbols are sent to the Q channel41. The 2-bit symbols from the outputs of the 1^(st) and 2^(nd) formatconverters 35 and 36 enter switches S3 and S4, respectively. Switch S3controlled by signals from the direct output O of the SAOZ generator 37(see FIG. 6 c) allows passage only odd symbols (see FIG. 6 d). At thesame time, switch S4 controlled by signals from the inverse output O ofthe SAOZ generator 37 (see FIG. 6 f) allows passage only even symbols(see FIG. 6 g). As shown in FIG. 6 d, the stream of 2-bit symbols fromthe output of switch S3 in the I channel 39 consists of odd symbolscorresponding to the modulator input data bits (these symbols can beequal to 1 or −1) and even symbols corresponding to the spaces (thesesymbols are equal to 0). At the same time, as shown in FIG. 6 g, thestream of 2-bit symbols from the output of switch S4 in the Q channel 41consists of even symbols corresponding to the modulator input data bits(these symbols can be equal to 1 or −1) and odd symbols corresponding tothe spaces (these symbols are equal to 0). Consequently, all evensymbols in the I channel 39 and all odd symbols in the Q channel 41 areequal to 0. Each symbol in any channel has the same length equal toT_(b).

The 2-bit symbols from the output of switch S3 in the I channel 39 andfrom the output of switch S4 in the Q channel 41 separately undergodifferential polarity modulation in first and second differentialpolarity modulators 42 and 43. Denoting 2-bit symbols at the input of adifferential modulator by a_(k) and 2-bit symbols at its output byb_(k), where k is a symbol number, the differential polarity modulationis as follows:b ₁ =a ₁ , b ₂ =a ₂, and b _(k) =a _(k) ×b _(k−2) for k≧3.

In both I and Q channels 39 and 41, all symbols b_(k) for k≧3 are formedby the 1^(st) and 2^(nd) differential modulators (see FIG. 3). Thesedifferential modulators are identical and each of them comprises a timedelay element, for example, a digital memory, intended to provide 2T_(b)delay for its output symbols and a multiplier, for example, a digitalmultiplier. Specifically, the first differential modulator 42 comprisesa multiplier 47 and a time delay element 48 that provides a 2T_(b) timedelay between its input and output. The multiplier 47 has two inputs andan output. One input of the multiplier 47 is connected to the output ofthe format converter 35 coming through switches S3 and S5. The otherinput of the multiplier 47 is connected to the output of the time delayelement 48. The output of the multiplier 47 is connected to the input ofthe time delay element 48 and to the input of switch S7. Similarly, thesecond differential modulator 43 comprises a multiplier 49 and a timedelay element 50 that provides a 2T_(b) time delay between its input andoutput. The multiplier 49 has two inputs and an output. One input of themultiplier 49 is connected to the output of the format converter 36coming through switches S4 and S6. The other input of the multiplier 49is connected to the output of the time delay element 50. The output ofthe multiplier 49 is connected to the input of the time delay element 50and to the input of switch S8. The differential modulators 42 and 43 arevery simple units because each of their input and output symbols isrepresented by only two bits and multiplications can be easilyperformed. Indeed, since 2-bit symbols corresponding to the data bitscan be equal only 1 or −1, their product is equal to 1 when the symbolshave the same polarity, and it is equal to −1 when they have oppositepolarity. When 2-bit symbols corresponding to spaces are multiplied theproduct is always equal to 0.

The differential modulators 42 and 43 shown in FIG. 3 cannot formsymbols b₁ and b₂. In the I channel 39, b, corresponds to the 1^(st)input data bit, and b₂ corresponds to the space. In the Q channel 41, b₁corresponds to the space, and b₂ corresponds to the 2^(nd) input databit. To avoid loss of the first two input data bits, switches S5 and S6,operated by control unit 25, pass the first two 2-bit symbols to thedifferential modulators 42 and 43 and simultaneously to the buses thatbypass the differential modulators 42 and 43. Such connections areprovided only during the time interval equal to 2T_(b) after the startof the data transmission mode. Just after the end of this time interval,switches S5 and S6 pass 2-bit symbols only to the inputs of thedifferential modulators 42 and 43.

Thus, during the entire synchronization mode and the time interval equalto 2T_(b) at the beginning of the data transmission mode, switches S7and S8 connect buses that bypass the differential modulators 42 and 43to the LPFs 38 and 40 in the I and Q channels 39 and 41, respectively.During the rest of the data transmission mode, switches S7 and S8connect the outputs of the differential modulators 42 and 43 to the LPFs38 and 40 in the I and Q channels 39 and 41, respectively. The datasymbol stream at the output of switch S7 (I channel) during the datatransmission mode is shown in FIG. 6 e, and the data symbol stream atthe output of switch S8 (Q channel) during this mode is shown in FIG. 6h.

The data symbol streams from the outputs of switches S7 and S8 enter thepulse-shaping LPFs 38 and 40 in the I and Q channels 39 and 41.Comparison of the timing diagrams in FIGS. 6 e and 6 h with the timingdiagrams in FIGS. 4 d and 4 h shows that signals at the inputs of theLPFs 38 and 40 in the I and Q channels 39 and 41 have the same format inboth synchronization and data transmission modes. Therefore, the signalconstellation shown in FIG. 5 is equally related to both these modes.

In the data transmission mode, the LPFs 38 and 40 and the block 23 inthe TX signal path 24 perform the same processing as in thesynchronization mode. As a result of this processing, a real-valuedbandpass digital signal corresponding to the input data bits is formed.This signal is translated into the analog domain by a digital to analogto converter D/A, and, after further analog processing in the TX signalpath 24, is transmitted by the antenna 12.

For the same reason as in the synchronization mode, only +90° or −90°gradual phase transitions between adjacent symbols are possible in thereal-valued bandpass digital signal corresponding to the input data bitsformed during the data transmission mode. Therefore, all the amplifiersin the TX signal path including the PA can operate in nonlinear mode(for example, as class C amplifiers) that allows efficient utilizationof transmitter power. At the same time, the data are transmitted usingbinary 0° and 180° phase differences between adjacent in time sameparity output symbols. This provides high noise immunity ofcommunications in the AWGN channels in addition to efficient utilizationof transmitter power.

The order of the operations in the 1^(st) embodiment of sync sequencegenerator and modulator 22 can be changed. For example, modulator inputdata bits can be converted into 2-bit symbols before separation of theodd and even bits. Selection of the appropriate halves of bits of the1^(st) and 2^(nd) PN sequences can be performed before the formatconversions. These and some other changes in the order of operation donot influence the system performance.

The 2^(nd) Embodiment of Sync Sequence Generation and Modulation

The distinctive features of the 2^(nd) embodiment of sync sequencegeneration and modulation compared to other embodiments are jointdifferential encoding of the same parity input data bits for the I and Qchannels 39 and 41 and separate format conversion at the input of theLPFs 38 and 40 in these channels. Compared to the 1^(st) embodimentwhere ±90° phase transitions between adjacent symbols are achieved dueto processing odd symbols in the I channel 39 and even symbols in the Qchannel 41 or vice versa, the phase transitions between adjacent symbolsin the 2^(nd) embodiment are limited to ±90° due to elimination ofsimultaneous transitions of the symbols' polarity in the I and Qchannels 39 and 41, while all symbols are processed in both I and Qchannels 39 and 41.

The block diagram that illustrates operation of the sync sequencegenerator and modulator 22 of this embodiment is shown in FIG. 7.Similar to the block diagram shown in FIG. 3, the block diagram in FIG.7 contains an FEC encoder 20 and an input block 23 of the TX signal path24 (this block forms a real-valued bandpass digital signal) in additionto the sync sequence generator and modulator 22, which is directlyinfluenced by this embodiment of the present invention. Transition fromsynchronization mode to data transmission mode is provided by switch S3operated by a control unit 25 in FIG. 7.

During the synchronization mode, the 1^(st) and 2^(nd) PN sequences aregenerated by the first and second PN sequence generators 33 and 34 inFIG. 7, respectively. Requirements for these sequences and theirparameters in the 2^(nd) embodiment are the same as those in the 1^(st)embodiment. The timing diagrams illustrating operation of the 2^(nd)embodiment in the synchronization mode are shown in FIGS. 8 a-8 j. Thefirst 11 bits of the 1^(st) and 2^(nd) PN sequences are depicted in FIG.8 a and FIG. 8 b, respectively. The bits of the 1^(st) PN sequence enterswitch S1 controlled by signals from the direct output O of the SAOZgenerator 37 (see the timing diagrams of the signal at the output O inFIG. 8 c). As a result of the switch S1 operation, only the first halfof each bit passes through this switch (see FIG. 8 e). At the same time,the bits of the 2^(nd) PN sequence enter switch S2 controlled by signalsfrom the inverse output O of the SAOZ generator 37 (see the timingdiagrams of the signal at the output O in FIG. 8 d). As a result of theswitch S2 operation, only the second half of each bit passes throughthis switch (see FIG. 8 f).

Thus, spaces at the output of switch S1 coincide with the second halvesof the 2^(nd) PN sequence bits at the output of switch S2, and spaces atthe output of switch S2 coincide with the first halves of the 1^(st) PNsequence bits at the output of switch S1 (see FIGS. 8 e and 8 f). Thesignals from the outputs of switches S1 and S2 are combined in a logicaladder 51 (an OR gate). As a result of this combining, the real-valuedbinary sync sequence (see FIG. 8 g) is formed by alternating the 1^(st)and 2^(nd) PN sequences that have the same length, starting time, andbit rate 0.5R. The real-valued binary sync sequence (see FIG. 8 g)enters both I and Q channels 39 and 41 after passing through switch S3in FIG. 7. The differential encoder 52 does not operate and isdisconnected from other units by switch S3 during the synchronizationmode.

Although the real-valued binary sync sequence enters both I and Qchannels 39 and 41, the sequence is processed differently in thesechannels. In one of the channels, it is directly sent to the formatconverter. In another channel, the binary sync sequence is sent to theformat converter after modulo 2 adding the sync sequence to a series ofalternating ones and zeros from the SAOZ generator 37. In the version ofthe 2^(nd) embodiment shown in FIG. 7, modulo 2 adder 55 is installedprior to the first format converter 35 in the I channel 39 and thedirect output O of the SAOZ generator 37 is used. In the Q channel 41,the binary sync sequence is sent directly to the second format converter36. In principle, modulo 2 adding of the SAOZ to the sync sequence canbe carried out in any of the quadrature channels and any of two outputsof the SAOZ generator 37 can be used.

The timing diagram of the binary sync sequence obtained at the output ofthe modulo 2 adder 55, i.e. at the input of the 1^(st) format converter35 in the I channel is shown in FIG. 8 h, while the binary sync sequenceat the input of the 2^(nd) format converter 36 in the Q channel isreflected by the timing diagram in FIG. 8 g. In both format converters35 and 36, bits are mapped into 2-bit symbols according to the rule:1→−1 and 0→1. The stream of 2-bit symbols generated at the output of the1^(st) format converter 35 enters the pulse-shaping LPF 38 in the Ichannel 39 (see FIG. 8 i), and the stream of 2-bit symbols generated atthe output of the 2^(nd) format converter 36 enters the pulse-shapingLPF 40 in the Q channel 41 (see FIG. 8 j). The stream of 2-bit symbolsshown in FIG. 8 i is the I component of the complex-valued basebanddigital signal corresponding to the sync sequence, and the stream of2-bit symbols shown in FIG. 8 j is the Q component of this signal. Thecomponents are further processed in the pulse-shaping LPFs 38 and 40 ofthe I and Q channels 39 and 41, respectively. In the 2^(nd) embodiment,the LPFs 38 and 40 carry out the same functions as the LPFs 38 and 40 inthe 1^(st) embodiment.

FIGS. 8 i and 8 j show that change of the symbol polarity (transitionsfrom 1 to −1 or vice versa) at any boundary between adjacent symbols canhappen only in one of the channels 39 or 41. This means that phasetransitions between adjacent complex-valued symbols can be only +90° or−90°. Indeed, since 180° phase transitions between adjacent symbols canhappen only if both I and Q quadrature components simultaneously changetheir polarity, such transitions are impossible in this embodiment ofthe invention.

The signal constellation corresponding to the 2^(nd) embodiment of theinvention is shown in FIG. 9. Here, white circles correspond to evenbinary symbols, and black circles correspond to odd binary symbols. Asfollows from the timing diagrams shown in FIGS. 8 i and 8 j, transitionsbetween circles with the same color are impossible.

In the 2^(nd) embodiment, the block 23 in the TX signal path 24 performsthe same processing as in the 1^(st) embodiment. As a result of thisprocessing, a real-valued bandpass digital signal corresponding to thecomplex-valued sync sequence (constructed of two alternating PNsequences) is formed. The signal is translated into the analog domain bya digital to analog converter D/A, and, after further analog processingin the TX signal path 24, is transmitted by antenna 12. Since only ±90°gradual phase transitions between adjacent symbols are possible in thereal-valued bandpass digital signal, all the amplifiers in the TX signalpath 24 including the power amplifier PA can operate in nonlinear mode(for example, as class C amplifiers). This enables efficient utilizationof the transmitter power.

During the data transmission mode, the 1^(st) and 2^(nd) PN sequencesare not generated, and the encoded data from the output of the FECencoder 20 enter the sync sequence generator and modulator 22 with bitrate R. The timing diagrams illustrating operation of the 2^(nd)embodiment in the data transmission mode are shown in FIGS. 10 a-10 f.The input data (see FIG. 10 a) are differentially encoded first by adifferential encoder 52.

Denoting bits at the input of a differential encoder 52 by c_(k) andbits at its output by d_(k), where k is a bit number, this encoding canbe can described as follows:d ₁ =c ₁ , d ₂ =c ₂, and d _(k) =c _(k) ⊕d _(k−2) for k≧3where ⊕ means modulo 2 summing. Thus, the differential encoder 52 is avery simple unit that contains time delay element 56, for example, a2-bit memory, and a modulo 2 adder 57. One input of the adder 57 isconnected to the output of the FEC encoder 20. The other input of theadder 57 is connected to the output of the time delay element 56. Theoutput of the adder 57 is connected to the input of the time delayelement 56 and to the data input of switch S3. The bit stream ofdifferentially encoded input data is shown in FIG. 10 b.

After passing through switch S3, this bit stream enters both I and Qchannels 39 and 41, where it is processed exactly like the sync sequence(see FIG. 7). In the I channel 39, the bit stream undergoes modulo 2adding to the sequence of ones and zeros from the inverse output O ofthe SAOZ generator 37 (see FIG. 10 c), and the result (see FIG. 10 d) issent to the 1^(st) format converter 35. In the Q channel 41, the bitstream from the output of switch S3 directly enters the 2^(nd) formatconverter 36. In both format converters 35 and 36, bits are mapped into2-bit symbols according to the rule: 1→−1 and 0→1.

The stream of 2-bit symbols generated at the output of the 1^(st) formatconverter 35 enters the pulse-shaping LPF 38 in the I channel 39 (seeFIG. 10 e), and the stream of 2-bit symbols generated at the output ofthe 2^(nd) format converter 36 enters the pulse-shaping LPF 40 in the Qchannel 41 (see FIG. 10f). The signals at the inputs of the LPFs 38 and40 in the I and Q channels 39 and 41 are respectively the I and Qcomponents of a complex-valued baseband digital signal corresponding toinput data. In the data transmission mode, the pulse-shaping LPFs 38 and40 in the I and Q channels 39 and 41 operate exactly the same way as inthe synchronization mode.

As demonstrated by the timing diagrams in FIGS. 10 e and 10 f, changesof the symbol polarity at any boundary between adjacent symbols canhappen only in one of quadrature channels (in the I channel 39 or in theQ channel 41). Therefore, phase transitions between adjacentcomplex-valued symbols can be only +90° or −90°, and 180° phasetransitions between these symbols are impossible. The signalconstellation shown in FIG. 9 corresponds both to the synchronizationand the data transmission modes in the 2^(nd) embodiment of theinvention shown in FIG. 7.

Since only +90° or −90° gradual phase transitions between adjacentsymbols are possible during both synchronization and data transmissionmodes, the 2^(nd) embodiment enables the same efficient utilization ofthe transmitter power as the 1^(st) embodiment. At the same time, thedata is transmitted using binary phase differences between symbols ofthe same parity. These differences can be only 0° or 180°. Therefore,both 1^(st) and 2^(nd) embodiments of sync sequence generation andmodulation have equally high noise immunity in the AWGN channels inaddition to efficient utilization of transmitter power. At the sametime, the 2^(nd) embodiment requires fewer gates for its realizationthan the 1^(st) embodiment (compare block diagrams in FIG. 3 and FIG.7).

Comparison of the signal constellations in FIGS. 5 and 9 shows that thesignal constellation corresponding to the 2^(nd) embodiment can beshifted by ±45° or ±135° relative to the constellation of the 1^(st)embodiment. Since propagation of radio waves in any communicationchannel always introduces a certain phase shift in the transmittedsignal, a rotation in the signal constellation about any angle has noimpact on the receiver design and communication system performance.

The 3^(rd) Embodiment of Sync Sequence Generation and Modulation

The 3^(rd) embodiment of sync sequence generation and modulation is muchcloser to the 2^(nd) embodiment than to the 1^(st) one. In both 2^(nd)and 3^(rd) embodiments, phase transitions between adjacent symbols arelimited to ±90° due to elimination of simultaneous transitions of thesymbols' polarity in the I and Q channels, while all symbols areprocessed in both I and Q channels. The 3^(rd) embodiment carries outjoint differential encoding of the same parity input data bits for the Iand Q channels 39 and 41 exactly like the 2^(nd) embodiment. At the sametime, there are the following differences between the 2^(nd) and 3^(rd)embodiments. The 3^(rd) embodiment performs joint format conversion ofthe sync sequence and differentially encoded input data bits for the Iand Q channels, while the 2^(nd) embodiment performs separate formatconversion for these channels. In the 3^(rd) embodiment, the SAOZ isapplied to one of the quadrature channels after its conversion to 2-bitformat, while it is applied to the channel in binary format in the2^(nd) embodiment.

The block diagram that illustrates operation of the sync sequencegenerator and modulator 22 of the 3^(rd) embodiment is shown in FIG. 11.Similar to the block diagrams shown in FIGS. 3 and 7, this block diagramcontains an FEC encoder 20 and an input block 23 to the TX signal path24 (the input block 23 forms a real-valued bandpass digital signal as inFIG. 3) in addition to the sync sequence generator and modulator 22.Transition from the synchronization mode to the data transmission modeis provided by switch S3 operated by a control unit 25, as in the 2^(nd)embodiment. In the 3^(rd) embodiment, all the operations carried out byunits preceding switch S3 are the same and performed in the same orderas in the 2^(nd) embodiment. Comparison of the block diagrams shown inFIGS. 7 and 11 confirms this.

The timing diagrams illustrating operation of the 3^(rd) embodiment inthe synchronization mode are shown in FIGS. 12 a-12 j. The real-valuedbinary sync sequence at the outputs of the adder 51 and switch S3 isformed in the 3^(rd) embodiment exactly the same way as in the 2^(nd)embodiment. Therefore, the timing diagrams in FIGS. 12 a to 12 gcoincide with the timing diagrams in FIGS. 8 a to 8g. However, furtherprocessing in the 3^(rd) embodiment differs from that in the 2^(nd)embodiment.

In the 3^(rd) embodiment, the SAOZ from the direct output O of the SAOZgenerator enters the 1^(st) format converter 35 before being sent to theI channel 39, and the binary sync sequence enters the 2^(nd) formatconverter 36 before being sent to both I and Q channels 39 and 41. Inboth format converters 35 and 36, bits are mapped into 2-bit symbolsaccording to the rule: 1→−1 and 0→1. The stream of 2-bit symbols at theoutputs of the 1^(st) format converter 35 is shown in FIG. 12 h.

The stream of 2-bit symbols from the output of the 2^(nd) formatconverter 36 that corresponds to the sync sequence enters a multiplier60 in the I channel 39 and the LPF 40 in the Q channel 41. The Icomponent of the complex-valued baseband digital signal corresponding tothe sync sequence is formed as a result of multiplying the 2-bit symbolsfrom the output of the 2^(nd) format converter 36 by the 2-bit symbolsfrom the output of the 1^(st) format converter 35. The I component isshown in FIG. 12 i. It enters the LPF 38 in the I channel 39. The Qcomponent of the complex-valued baseband digital signal corresponding tothe sync sequence is the 2-bit symbol stream from the output of the2^(nd) format converter 36 that enters the LPF 40 in the Q channel 41.This 2-bit symbol stream is shown in FIG. 12 j. In the 3^(rd)embodiment, the LPFs 38 and 40 carry out the same functions as the LPFs38 and 40 in the 1^(st) and 2^(nd) embodiments.

The I component of the complex-valued baseband digital signal (see FIG.12 i) is a product of the multiplication of the signal Q component (seeFIG. 12 j) by the 2-bit symbols corresponding to the SAOZ from thedirect output O of the SAOZ generator 37 (see FIG. 12 h). The Qcomponent of the complex-valued baseband digital signal (see FIG. 12 j)can be considered a product of the multiplication of the I signalcomponent (see FIG. 12 i) by the 2-bit symbols corresponding to the SAOZfrom the direct output O of the SAOZ generator 37 (see FIG. 12 h). Thisconfirms the fact that, in principle, multiplying the 2-bit symbols fromthe output of the 2^(nd) format converter 36 by the 2-bit symbols fromthe output of the 1^(st) format converter 35 can be performed in any ofthe quadrature channels.

The timing diagram shown in FIG. 12 i is the same as that shown in FIG.8 i, and the timing diagram shown in FIG. 12 j is the same as that shownin FIG. 8 j. Since these diagrams are identical, the signalconstellations for the 2^(nd) and 3^(rd) embodiments are also identical.Taking into account that signal processing in the TX signal path 24 isthe same for all three embodiments, the conclusion can drawn that only±90° gradual phase transitions between adjacent symbols are possible inthe bandpass signal corresponding to the sync sequence at thetransmitter output. This enables efficient utilization of thetransmitter power.

The timing diagrams illustrating operation of the 3^(rd) embodiment inthe data transmission mode are shown in FIGS. 13 a to 13 f. During thismode, the 1^(st) and 2^(nd) PN sequences are not generated, and theencoded data from the output of the FEC encoder 20 enter the syncsequence generator and modulator 22 with bit rate R (see FIG. 13 a). Themodulator input data are differentially encoded exactly as in the 2^(nd)embodiment (see FIG. 13 b), and the same SAOZ from the direct output Oof the SAOZ generator 37 (see FIG. 13 c) is used for further processing.Therefore, the timing diagrams in FIGS. 13 a, 13 b, and 13 c coincidewith the timing diagrams in FIGS. 10 a, 10 b, and 10 c, respectively.

Consequently, in the 2^(nd) and 3^(rd) embodiments, the same bit streamof differentially encoded modulator input data passes through switch S3during the data transmission mode. In the 3^(rd) embodiment, thedifferentially encoded input data during the data transmission mode areprocessed exactly the same way as the sync sequence during thesynchronization mode. Thus, processing of the differentially encodedinput data is different in the 2 d and 3^(rd) embodiments. The stream of2-bit symbols at the output of the 1^(st) format converter 35 thatcorresponds to the SAOZ from the direct output O of the SAOZ generator37 is shown in FIG. 13 d. The I component of the complex-valued basebanddigital data signal at the input of the LPF 38 in the I channel 39 isobtained as a result of multiplying 2-bit symbols from the outputs ofthe 1^(st) and 2^(nd) format converters 35 and 36 in the multiplier 60.This component is shown in FIG. 13 e. The Q component of thecomplex-valued baseband digital data signal at the input of the LPF 40in the Q channel 41 is the output signal of the 2^(nd) format converter36. This component is shown in FIG. 13 f.

The timing diagram shown in FIG. 13 e is identical to the diagram shownin FIG. 10 e, and the timing diagram shown in FIG. 13 f is identical tothe diagram shown in FIG. 10 f. The identities of the timing diagramsshown in FIGS. 10 e and 13 e and FIGS. 10 f and 13 f, as well as theidentity of the timing diagrams shown in FIGS. 8 i and 12 i and FIGS. 8j and 12 j prove that, although the 2^(nd) and 3^(rd) embodimentsprocess signals differently after switch S3, they provide the sameresults.

These identities also confirm that the 2^(nd) and 3^(rd) embodimentshave identical signal constellations and identical properties of theoutput signals. Comparison of the block diagrams shown in FIGS. 7 and 11proves that the 2^(nd) and 3^(rd) embodiments provide equally simplerealization.

Synchronization and Demodulation

As has been shown above, all three embodiments of modulation and syncsequence generation produce identical signals at the receiver input.Therefore, the same methods and apparatus for synchronization anddemodulation can be used in the RX part 28 of the modem for anyembodiment of the sync sequence generator and modulator 22.

FIG. 14 is a block diagram that illustrates the structure and operationof one example of the synchronizer and demodulator 30. In addition tothe synchronizer and demodulator 30, FIG. 14 also shows the last block65 of the RX signal path 26 and the FEC decoder 32. They are shown toclarify the succession of signal processing in the receiver RX. At thesame time, as mentioned above, the proposed AQDBPSK demodulation can becombined with any method of decoding and with any architecture of the RXsignal path 26.

Although there are many ways of signal processing in the RX signal path26 and all of them are compatible with the invention, the most effectiveway for any type of modulation and demodulation is as follows. In the RXsignal path 26, a received real-valued bandpass analog signal (summedwith noise and interference from the communication channel) isamplified, filtered, and converted to an intermediate frequency (IF). Asshown in FIG. 2, the beginning of each transmission corresponds to thesync sequence, and the rest of transmission corresponds to the encodedand modulated data. At the IF, the received signal is translated intothe digital domain by an analog-to-digital converter (A/D). Thereal-valued digital bandpass signal from the A/D enters the last block65 of the RX signal path 26. Block 65 digitally forms in-phase andquadrature components simultaneously converting the A/D output signal tozero frequency. This is achieved by multiplying the A/D output signal bysine and cosine whose frequencies are equal to the center frequency ofthe A/D output signal and filtering the products by two identicaldigital lowpass filters (LPFs) 70 and 71. The multiplications areperformed by multipliers 67 and 68. As a result of the multiplicationsand filtering, the signal at the output of the block 65 is acomplex-valued baseband digital signal. This signal represented bymultibit samples of its I and Q components is an input signal of thesynchronizer and demodulator 30.

During a synchronization mode, it is necessary to decide if there is asignal intended for this receiver and to accurately determine the end ofthe sync sequence. This information is sufficient for symbol and wordsynchronization because data transmission starts immediately after syncsequence, and the lengths of the symbols and code words are known at thereceiver RX of the communication system.

The complex-valued digital signal from the output of block 65 is dividedinto two branches directly at the input of the synchronizer anddemodulator 30. The first branch is comprised of a complex-valuedmatched filter 73, peak selector 75, and generator of clock pulses 77.This branch carries out identification of the sync sequence andsynchronization. The second branch is comprised of all other blocks ofthe synchronizer and demodulator 30. The complex-valued digital signalcan enter the 1^(st) and 2^(nd) delay elements 80 and 81, but it cannotpenetrate further in the second branch until synchronization iscompleted because switches S10 and S11 disconnect all subsequent blocksfrom the outputs of the 1^(st) and 2^(nd) delay elements 80 and 81 atthe beginning of reception.

Various synchronization techniques can be used in the invention. Thefastest synchronization, which is usually required for bursttransmissions, can be achieved by using a complex-valued matched filter73. The filter 73 is matched to the sync sequence generated in thetransmitter TX. Consequently, it can identify the sync sequence and itsoutput signal reaches a maximum level exactly at the end of the syncsequence. In practice, however, determining an accurate position of theend of sync sequence is complicated by the fact that each symbol in thefilter 73 is usually represented by relatively small number of samplesto minimize the required speed of signal processing. Therefore, somekind of interpolation is required to accurately determine the end ofsync sequence. This interpolation and accurate determining the positionof the sample that corresponds to the end of sync sequence is performedby the peak selector 75. There are many known methods of theseprocedures and virtually any of them can be used in the invention.

When the end of sync sequence is determined, peak selector 75 sendscontrol signals to the clock pulse generator 77, switches S10 and S11,and tracking system 97. These signals initiate demodulation mode of thesynchronizer and demodulator 30. Immediately after receiving the controlsignal from the peak selector 75, the clock pulse generator 77 startsgenerating all clock pulses necessary for demodulation and decoding. Thecontrol signal sent to switches S10 and S11 causes the switches toconnect the outputs of the 1^(st) and 2^(nd) delay elements 80 and 81 tothe inputs of integrators 83 and 84, respectively, and they stayconnected until the end of signal reception. The control signal sent tothe tracking system 97 initializes its operation.

Connections provided by the switches S10 and S11 allow thecomplex-valued digital signal from the output of the block 65 to enterthe integrators 83 and 84 after passing through the 1^(st) and 2^(nd)delay elements 80 and 81. The 1^(st) and 2^(nd) delay elements 80 and 81are identical. They are necessary for compensation of the delay causedby signal processing in the peak selector 75. Signals that enter theintegrators 83 and 84 are digital samples of the symbols that correspondto the received data. The integrators 83 and 84 carry out digitalsumming of the signal samples during the time T_(b) equal to the symbollength. This summing is performed under control of the generator ofclock pulses 77.

In the I channel, the output signal of the integrator 83 simultaneouslyenters a 2T_(b) delay element 87 and a first input of the multiplier 90.The output signal of the 2T_(b) delay element 87 enters a second inputof the multiplier 90. The 2T_(b) delay element 87 and the multiplier 90perform differential demodulation of same parity multibit symbols in theI channel. Denoting multibit symbols at the output of the integrator 83by i_(k) and multibit symbols at the output of the multiplier 90 byβ^(i) _(k), where k is a symbol number, we can describe the differentialdemodulation as follows:β^(i) ₁ =i ₁, β^(hd 3) =i ₂, and β^(i) _(k) =i _(k) × _(k−2) for k≧3.To correctly process symbols i₁ and i₂, two ones have to be written inthe 2T_(b) delay element 87 before the start of data demodulation.

In the Q channel, a 2T_(b) delay element 88 and a multiplier 91 processthe output signal of the integrator 84 the same way as the 2T_(b) delayelement 87 and the multiplier 90 process the output signal of theintegrator 83 in the I channel. As a result, differential demodulationof same parity multibit symbols in the Q channel is also performed.Denoting multibit symbols at the output of the integrator 84 by q_(k)and multibit symbols at the output of the multiplier 91 by β^(q) _(k),where k is a symbol number, we can describe the differentialdemodulation as follows:β^(q) ₁ =q ₁, β^(q) ₂ =q ₂, and β^(q) _(k) =q _(k) ×q _(k−2) for k≧3.

The output signals of the multipliers 90 and 91 are summed by an adder93. The output signal of the adder 93 is α*_(k)=β^(i) _(k)+β^(q) _(k).This signal enters a decision unit 95. When soft-decision demodulationis performed, the decision unit 95 forms a multibit output signal usingsign and value of α*_(k). When hard-decision demodulation is performed,the decision unit 95 forms a binary output signal a*_(k) using only signof α*_(k). FIG. 14 illustrates the case when hard-decision demodulationis carried out. In any case, the output signal of the decision unit 95is an output signal of the synchronizer and demodulator 30, and itenters the FEC decoder 32 where it is decoded. The demodulated anddecoded output data are sent to line 16. The described abovedemodulation procedure is optimal because it provides the highest noiseimmunity that can be achieved by noncoherent demodulation of binarysignals.

Tracking system 97 starts operation immediately after achievingsynchronization. Its task is to maintain time alignment between incomingsignal and generator of clock pulses 77. In principle, any existingtracking method can be used in the invention.

Thus, demodulation of the received AQDBPSK signals represented by its Iand Q components is performed by separately integrating the signalsduring the time equal to the symbol length, and multiplying a currentoutput signal of the integrator by the signal delayed by the time equalto the length of two modulator input bits in both I and Q channels,summing the products obtained in both channels, and making hard or softdecision about the received symbol based on the sign or sign and valueof the sum.

The complex-valued matched filter 73 used for synchronization differsfrom the prior art. It contains two identical real-valued filtersmatched to the 1^(st) sequence, consisting of the selected in themodulator halves of the symbols corresponding to the 1^(st) PN sequenceand spaces between the adjacent in time halves, and two real-valuedfilters matched to the 2^(nd) sequence, consisting of the selected inthe modulator halves of the symbols corresponding to the 2^(nd) PNsequence and spaces between the adjacent in time halves. An input signalfrom the I channel (i.e. an output signal of the I channel of the RXsignal path 26) enters simultaneously two real-valued finite impulseresponse (FIR) filters, one of which is matched to the 1^(st) sequence,consisting of the selected in the modulator halves of the symbolscorresponding to the 1^(st) PN sequence and spaces between the adjacentin time halves, and the other of which is matched to the 2^(nd)sequence, consisting of the selected in the modulator halves of thesymbols corresponding to the 2^(nd) PN sequence and spaces between theadjacent in time halves. Similarly, an input signal from the Q channel(i.e. an output signal of the Q channel of the RX signal path 26) enterssimultaneously two additional real-valued FIR filters, one of which ismatched to the 1^(st) sequence, consisting of the selected in themodulator halves of the symbols corresponding to the 1^(st) PN sequenceand spaces between the adjacent in time halves, and another is matchedto the 2^(nd) sequence, consisting of the selected in the modulatorhalves of the symbols corresponding to the 2^(nd) PN sequence and spacesbetween the adjacent in time halves. The real-valued matched filters canhave various forms of realization, for example, direct or transposedforms.

A block diagram of an example of the complex-valued matched filter 73with direct form of realization of real-valued matched FIR filters isshown in FIG. 15. Here, two real-valued matched filters, whose inputsare connected to the output of the I channel of the RX signal path 26,use the same I channel digital tapped delay line 100. Simultaneously,two real-valued matched filters, whose inputs are connected to theoutput of the Q channel of the RX signal path 26, use the same Q channeldigital tapped delay line 101. The digital tapped delay lines 100 and101 are identical and each of them consists of digital delay elements(for example, registers) 105. Each of the digital delay elements 105provides delay equal to the sampling interval T_(s). The distancebetween neighboring taps is equal the symbol length T_(b). Thus, thenumber of the digital delay elements 105 between each pair ofneighboring taps is equal to the ratio T_(b)/T_(s). In the example ofthe complex-valued matched filter 73 shown in FIG. 15, T_(b)/T_(s)=4.This means that each symbol is represented by 4 samples, and 4 digitaldelay elements 105 should be placed between each pair of neighboringtaps. All even taps of the digital tapped delay lines 100 and 101(counting from left to right) belong to the filter matched to the 1^(st)sequence, consisting of the selected in the modulator halves of thesymbols corresponding to the 1^(st) PN sequence and spaces between theadjacent in time halves, and all odd taps of these lines belong to thefilter matched to the 2^(nd) sequence, consisting of the selected in themodulator halves of the symbols corresponding to the 2^(nd) PN sequenceand spaces between the adjacent in time halves. In each tap of thedigital tapped delay lines 100 and 101, a multiplier is installed. Thismultiplier multiplies the signal in the tap by coefficients of the1^(st) or 2^(nd) sequences. The coefficients can be +1 or −1. Taps withcoefficients equal to 0 are excluded. In FIG. 15, the multipliers 107belonging to the filters matched to the 1^(st) sequence, consisting ofthe selected in the modulator halves of the symbols corresponding to the1^(st) PN sequence and spaces between the adjacent in time halves, arereflected by squares with the value of the coefficient (+1 or −1)inside, and the multipliers 108 belonging to the filters matched to the2^(nd) sequence, consisting of the selected in the modulator halves ofthe symbols corresponding to the 2^(nd) PN sequence and spaces betweenthe adjacent in time halves, are reflected by circles with the value ofthe coefficient (+1 or −1) inside. Technically multipliers 107 and 108are identical.

Thus, the complex-valued matched filter convolutes the in-phasecomponent of the received synchronization sequence with the impulseresponses of the first and second real-valued filters. The I componentof the input signal of the complex-valued matched filter 73 passes overthe digital tapped delay line 100 and its taps to the multipliers 107and 108, where this component is multiplied by appropriate coefficients.The output signals of all the multipliers 107 connected to the I channeldigital tapped delay line 100 are summed by the 1^(st) sequence adder110, and the output signals of all the multipliers 108 connected to thisline are summed by the 2^(nd) sequence adder 114.

The complex-valued matched filter 73 likewise convolutes the quadraturecomponent of the received synchronization sequence with impulseresponses of third and fourth real-valued filters. The Q component ofthe input signal of the complex-valued matched filter 73 passes over thedigital tapped delay line 101 and its taps to the multipliers 107 and108, where this component is also multiplied by appropriatecoefficients. The output signals of all the multipliers 107 connected tothe Q channel digital tapped delay line 101 are summed by the 1^(st)sequence adder 11 1, and the output signals of all the multipliers 108connected to this line are summed by the 2^(nd) sequence adder 115. Theoutputs of the 1^(st) sequence adders 110 and 111 are the outputs of thefilters matched to the 1^(st) sequence, and the outputs of the 2^(nd)sequence adders 114 and 115 are the outputs of the filters matched tothe 2^(nd) sequence.

Signals from the output of the 1^(st) sequence adder 110 and from theoutput of the 2^(nd) sequence adder 115 are summed with the same signsby the adder 118. This forms the I component of the output signal of thecomplex-valued matched filter 73. Signals from the output of the 2^(nd)sequence adder 114 with inverted sign and from the output of the 1^(st)sequence adder 111 are summed by the adder 119. In other words, thesignal from the output of the 2^(nd) sequence adder 114 is subtractedfrom the signal from the output of the 1^(st) sequence adder 111. Thisforms the Q component of the output signal of the complex-valued matchedfilter 73.

The I and Q components of the output signal of the complex-valuedmatched filter 73 (i.e. the output signals of the adders 118 and 119)are separately squared by multipliers 122 and 123 respectively. Thesquared components are summed by the adder 125. The output signals ofthe adder 125 are output signals of the complex-valued matched filter73.

Thus, realization of the complex-valued matched filter 73 requires twoidentical real-valued filters matched to the 1^(st) sequence, consistingof the selected in the modulator halves of the symbols corresponding tothe 1^(st) PN sequence and spaces between the adjacent in time halves,and two real-valued filters matched to the 2^(nd) sequence, consistingof the selected in the modulator halves of the symbols corresponding tothe 2^(nd) PN sequence and spaces between the adjacent in time halves.One filter matched to the 1^(st) sequence and one filter matched to the2^(nd) sequence is connected to the I channel input of thecomplex-valued matched filter 73, and another filter matched to the1^(st) sequence and another filter matched to the 2^(nd) sequence isconnected to the Q channel input of the complex-valued matched filter73.

When a direct form of realization of real-valued matched FIR filters isemployed, the same digital tapped delay line is used for bothreal-valued FIR filters in each of quadrature channels. This radicallysimplifies implementation of the invention.

Properties of the Proposed Methods and Apparatus

As follows from the description above, all three preferable embodimentsof the modulation and sync sequence generation limit possible phasetransitions between adjacent symbols to ±90° and make these transitionsgradual (smooth). This enables efficient utilization of transmitterpower because such transitions allow operation of all the amplifiers ofthe transmitter analog signal path including the power amplifier PA inenergy efficient nonlinear mode without any significant regeneration ofthe signal spectrum side lobes.

Simultaneously, in all the embodiments, the modulator input data aretransmitted by binary 0° and 180° phase differences between same paritymodulator output symbols. Preferably, optimal noncoherent demodulationof the received same parity symbols is used. This provides high noiseimmunity of communications in the AWGN channels in addition to efficientutilization of transmitter power.

Further increases in overall energy efficiency of the communicationsystem are attained due to two factors.

First, the proposed modulation method allows noise immune noncoherentdemodulation that reduces synchronization time. Fast synchronizationsignificantly lowers energy consumption during burst transmissions.

Second, methods of modulation, noncoherent demodulation, andsynchronization employed in the system can easily be realized using thesimplest digital signal processing operations and the simplest digitalcircuits. These methods can easily be implemented in a universalreconfigurable digital modem. This enables significant reduction ofpower consumption in real world applications.

In addition, entirely digital realization of the method and apparatusmake them resistant to all destabilizing factors and simplifies theirpractical implementation, and fast and reliable synchronizationincreases time allocated for data transmission when transmission time islimited.

The principles of modulation, demodulation, and synchronizationdisclosed herein are applicable to communications using any form ofphysical information carrier, including, but not limited to, those usedin radio frequency or optical communications.

REFERENCES

-   [1] B. Sklar, Digital Communications, Fundamentals and Applications,    second edition, Prentice Hall PTR, Upper Saddle River, N.J., 2001.-   [2] J. G. Proakis, Digital Communications, fourth edition,    McGraw-Hill, Upper Saddle River, N.Y., 2001.-   [3] Fuqin Xiong, Digital Modulation Techniques, Artech House,    Boston/London, 2000.-   [4] E. A. Lee and D. G. Messerschmitt, Digital Communications,    second edition, Kluwer Academic Publishers, Boston/Dordrecht/London,    2001.-   [5] H. Yazdani, K. Feher, and W. Steenaart, FBPSK, “Constant    Envelope Bandlimited BPSK Signal.” IEEE Transaction on    Communication, vol. COM-28, No. 6, pp. 889-897, June 1980.-   [6] U.S. Pat. No 5,148,127. Int. Cl.⁵ H04L 27/20. U.S. Cl. 332/104;    375/55; 375/67. Date of Patent: Sep. 15, 1992. Byoung-Jin Cheon,    “Biphase Shift Keying Modulation Circuit Having Constant Envelope    Characteristics.”-   [7] U.S. Pat. No 5,491,457. Int. Cl.⁶ H03C 3/00; H04L 27/12; H04L    27/20. U.S. Cl. 332/103; 332/100; 375/302; 375/305. Date of Patent:    Feb. 13, 1996. Kamilo Feher, F-Modulation Amplification.-   [8] H. Mehdi and K. Feher, “FBPSK, Power and Spectrally Efficient    Modulation for PCS and Satellite Broadcasting Applications,” IEEE    Transaction on Broadcasting, vol. 42, No. 1, pp. 27-32, March 1996.

The Title, Technical Field, Background, Summary, Brief Description ofthe Drawings, Detailed Description, and Abstract are meant to illustratethe preferred embodiments of the invention and are not in any wayintended to limit the scope of the invention. The scope of the inventionis solely defined and limited in the claims set forth below. It isintended, however, that the claims not be limited to any particular formof electronic implementation. For example, any one or combination ofhardware, software, and/or firmware can be used to implement the claimedinvention. Also, although digital circuitry is preferred, analogcircuitry may be used in all or any part of specific implementationswithin the scope of the claims.

1. A method of performing alternating quadratures differential binaryphase shift keying modulation, comprising the steps of: limiting phasechanges between adjacent in time modulator output symbols to ±90°;transmitting modulator input data by 0° and 180° phase differencesbetween non-adjacent in time modulator output symbols; and producing asynchronization sequence and adding the synchronization sequence totransmitted data.
 2. The method of claim 1, in which the synchronizationsequence producing step comprises the steps of: generating a firstpseudorandom sequence; generating a second pseudorandom sequence;separately converting all bits of both pseudorandom sequences into 2-bitsymbols; selecting for further processing only the first in time half ofeach 2-bit symbol corresponding to the bits of the first pseudorandomsequence and only the second in time half of each 2-bit symbolcorresponding to the bits of the second pseudorandom sequence; forming acomplex-valued baseband synchronization sequence by sending the selectedfirst in time halves of the 2-bit symbols to one of an in-phase channelor a quadrature channel and the selected second in time halves of the2-bit symbols to the other of the in-phase channel or the quadraturechannel; and performing interpolating and pulse shaping digitalfiltering of the selected first in time halves of the 2-bit symbols inthe one channel and the selected second in time halves of the 2-bitsymbols in the other channel.
 3. The method of claim 1, in which thesynchronization sequence producing step comprises the steps of:generating a first pseudorandom sequence; generating a secondpseudorandom sequence; selecting for further processing only the firstin time half of each bit of the first pseudorandom sequence and only thesecond in time half of each bit of the second pseudorandom sequence;separately converting the selected first in time halves of bits and allthe selected second in time halves of bits of the pseudorandom sequencesinto 2-bit symbols; forming a complex-valued baseband synchronizationsequence by sending the 2-bit symbols corresponding to the selectedfirst in time halves of bits to one of an in-phase channel or aquadrature channel and the 2-bit symbols corresponding to the selectedsecond in time halves of bits to the other of the in-phase channel orthe quadrature channel; and performing interpolating and pulse shapingdigital filtering of the 2-bit symbols corresponding to the selectedfirst in time halves of bits in the one channel and the 2-bit symbolscorresponding to the selected second in time halves of bits in the otherchannel.
 4. The method of claim 1, in which the synchronization sequenceproducing step comprises the steps of: generating a first pseudorandomsequence; generating a second pseudorandom sequence; selecting forfurther processing only the first in time half of each bit of the firstpseudorandom sequence and the second in time half of each bit of thesecond pseudorandom sequence; forming a real-valued basebandsynchronization sequence by logically summing the selected first in timehalves and the second in time halves; sending the real-valued basebandsynchronization sequence to both an in-phase channel and a quadraturechannel; converting the bits of the real-valued baseband synchronizationsequence into 2-bit symbols in one of the in-phase or quadraturechannels; modulo 2 adding a sequence of alternating ones and zeros tothe bits of the real-valued baseband synchronization sequence in theother of the in-phase or quadrature channels; converting the modulo 2sums into 2-bit symbols; and performing interpolating and pulse shapingdigital filtering of the 2-bit symbols in both the in-phase andquadrature channels.
 5. The method of claim 1, in which thesynchronization sequence producing step comprises the steps of:generating a first pseudorandom sequence; generating a secondpseudorandom sequence; selecting for further processing only the firstin time half of each bit of the first pseudorandom sequence and only thesecond in time half of each bit of the second pseudorandom sequence;forming a real-valued baseband synchronization sequence by logicallysumming the selected first in time halves and second in time halves;converting the bits of the real-valued baseband synchronization sequenceinto 2-bit symbols; converting the bits of a sequence of alternatingones and zeros into 2-bit symbols; sending 2-bit symbols correspondingto the bits of the real-valued baseband synchronization sequence to bothin-phase and quadrature channels; multiplying the 2-bit symbolscorresponding to the bits of the real-valued baseband synchronizationsequence by the 2-bit symbols corresponding to the sequence ofalternating ones and zeros and obtaining 2-bit products in one of thechannels; performing interpolating and pulse shaping digital filteringof the 2-bit products in the channel where the multiplication is carriedout; and performing interpolating and pulse shaping digital filtering ofthe 2-bit symbols corresponding to the bits of the real-valued basebandsynchronization sequence in the other channel.
 6. The method of claim 2,in which the two pseudorandom sequences have the same number of bits,the same starting time, and a bit rate that is half the bit rate of theinput data stream.
 7. The method of claim 3, in which the twopseudorandom sequences have the same number of bits, the same startingtime, and a bit rate that is half the bit rate of the input data stream.8. The method of claim 4, in which the two pseudorandom sequences havethe same number of bits, the same starting time, and a bit rate that ishalf the bit rate of the input data stream.
 9. The method of claim 5, inwhich the two pseudorandom sequences have the same number of bits, thesame starting time, and a bit rate that is half the bit rate of theinput data stream.
 10. The method of claim 2, in which the selectionstep comprises the steps of: generating direct and inverse sequences ofalternating ones and zeros, at a rate equal to the data rate of themodulator input bit stream; and controlling a first switch connected tothe output of the first pseudorandom sequence generator by the directsequence of alternating ones and zeros and a second switch connected tothe output of the second pseudorandom sequence generator by the inversesequence of alternating ones and zeros.
 11. The method of claim 3, inwhich the selection step comprises the steps of: generating direct andinverse sequences of alternating ones and zeros, at a rate equal to thedata rate of the modulator input bit stream; and controlling a firstswitch connected to the output of the first pseudorandom sequencegenerator by the direct sequence of alternating ones and zeros and asecond switch connected to the output of the second pseudorandomsequence generator by the inverse sequence of alternating ones andzeros.
 12. The method of claim 4, in which the selection step comprisesthe steps of: generating direct and inverse sequences of alternatingones and zeros, at a rate equal to the data rate of the modulator inputbit stream; and controlling a first switch connected to the output ofthe first pseudorandom sequence generator by the direct sequence ofalternating ones and zeros and a second switch connected to the outputof the second pseudorandom sequence generator by the inverse sequence ofalternating ones and zeros.
 13. The method of claim 5, in which theselection step comprises the steps of: generating direct and inversesequences of alternating ones and zeros, at a rate equal to the datarate of the modulator input bit stream; and controlling a first switchconnected to the output of the first pseudorandom sequence generator bythe direct sequence of alternating ones and zeros and a second switchconnected to the output of the second pseudorandom sequence generator bythe inverse sequence of alternating ones and zeros.
 14. A method ofsynchronizing a demodulator of alternating quadratures differentialbinary phase shift keying signals, comprising the steps of: formingin-phase and quadrature components of a received synchronizationsequence; convoluting the in-phase component of a receivedsynchronization sequence with impulse responses of first and secondreal-valued filters, the first filter being matched to a first sequencecomprising selected in the modulator halves of symbols corresponding toa first pseudorandom sequence and spaces between the adjacent in timehalves, and the second filter being matched to a second sequencecomprising selected in the modulator halves of the symbols correspondingto a second pseudorandom sequence and spaces between the adjacent intime halves; convoluting the quadrature component of the receivedsynchronization sequence with impulse responses of third and fourthreal-valued filters, the third filter being matched to a sequencecomprising selected in the modulator halves of symbols corresponding toa first pseudorandom sequence and spaces between the adjacent in timehalves, and the fourth filter being matched to a sequence comprisingselected in the modulator halves of symbols corresponding to a secondpseudorandom sequence and spaces between the adjacent in time halves;forming an in-phase component of an output signal of the complex-valuedmatched filter comprising the four real-valued matched filters bysumming an output signal of the first real-valued filter connected tothe in-phase channel and an output signal of the fourth real-valuedfilter connected to the quadrature channel; forming a quadraturecomponent of the output signal of the complex-valued matched filtercomprising the four real-valued matched filters by subtracting an outputsignal of the second real-valued filter connected to the in-phasechannel from an output signal of the third real-valued filter connectedto the quadrature channel; forming a real-valued output signal of thecomplex-valued matched filter by separately squaring its output in-phaseand quadrature components and summing the squared components; anddetermining the correct position of clock pulses by selecting a peak ofthe real-valued output signal of the complex-valued matched filter. 15.An apparatus for performing alternating quadratures differential binaryphase shift keying modulation, comprising: a modulator that limits phasechanges between adjacent in time modulator output symbols to ±90°,transmits modulator input data by 0° and 180° phase differences betweennon-adjacent in time modulator output symbols, and produces asynchronization sequence and adds the synchronization sequence to thetransmitted data.
 16. The apparatus of claim 15, in which the modulatorfurther comprises: a first pseudorandom sequence generator; a secondpseudorandom sequence generator; a format converter that separatelyconverts all bits of both pseudorandom sequences into 2-bit symbols; acircuit that selects for further processing only the first in time halfof each 2-bit symbol corresponding to the bits of the first pseudorandomsequence and only the second in time half of each 2-bit symbolcorresponding to the bits of the second pseudorandom sequence; a circuitthat forms a complex-valued baseband synchronization sequence by sendingthe selected first in time halves of the 2-bit symbols to one of anin-phase channel or a quadrature channel and the selected second in timehalves of the 2-bit symbols to the other of the in-phase channel or thequadrature channel; and a first interpolating and pulse shaping digitalfilter of the selected first in time halves of the 2-bit symbols in theone channel and a second interpolating and pulse shaping digital filterof the selected second in time halves of the 2-bit symbols in the otherchannel.
 17. The apparatus of claim 15, in which the modulatorcomprises: a first pseudorandom sequence generator; a secondpseudorandom sequence generator; a circuit that selects for furtherprocessing only the first in time half of each bit of the firstpseudorandom sequence and only the second in time half of each bit ofthe second pseudorandom sequence; a format converter that separatelyconverts all the selected first in time halves of bits and all theselected second in time halves of bits of the pseudorandom sequencesinto 2-bit symbols; a circuit that forms a complex-valued basebandsynchronization sequence by sending the 2-bit symbols corresponding tothe selected first in time halves of bits to one of an in-phase channelor a quadrature channel and the 2-bit symbols corresponding to theselected second in time halves of bits to the other of the in-phasechannel or the quadrature channel; and a first interpolating and pulseshaping digital filter of the 2-bit symbols corresponding to theselected first in time halves of bits in the one channel and a secondinterpolating and pulse shaping digital filter of the 2-bit symbolscorresponding to the selected second in time halves of bits in the otherchannel.
 18. The apparatus of claim 15, in which the modulatorcomprises: a first pseudorandom sequence generator; a secondpseudorandom sequence generator; a circuit that selects for furtherprocessing only the first in time half of each bit of the firstpseudorandom sequence and the second in time half of each bit of thesecond pseudorandom sequence; a circuit that forms a real-valuedbaseband synchronization sequence by logically summing the selectedfirst in time halves and the second in time halves; a circuit that sendsthe real-valued baseband synchronization sequence to both an in-phasechannel and a quadrature channel; a format converter that converts thebits of the real-valued baseband synchronization sequence into 2-bitsymbols in one of the in-phase or quadrature channels; a modulo 2 adderthat adds a sequence of alternating ones and zeros to the bits of thereal-valued baseband synchronization sequence in the other of thein-phase or quadrature channels; a format converter that converts themodulo 2 sums into 2-bit symbols; and a first interpolating and pulseshaping digital filter in the in-phase channel and a secondinterpolating and pulse shaping digital filter in the quadrature channelthat filter 2-bit symbols in the in-phase and quadrature channelsrespectively.
 19. The apparatus of claim 15, in which the modulatorcomprises: a first pseudorandom sequence generator; a secondpseudorandom sequence generator; a circuit that selects for furtherprocessing only the first in time half of each bit of the firstpseudorandom sequence and only the second in time half of each bit ofthe second pseudorandom sequence; a circuit that forms a real-valuedbaseband synchronization sequence by logically summing the selectedfirst in time halves and second in time halves; a first format converterthat converts the bits of the real-valued baseband synchronizationsequence into 2-bit symbols; a second format converter that converts thebits of a sequence of alternating ones and zeros into 2-bit symbols; acircuit that sends 2-bit symbols corresponding to the bits of thereal-valued baseband synchronization sequence to both in-phase andquadrature channels; a multiplier that multiplies the 2-bit symbolscorresponding to the bits of the real-valued baseband synchronizationsequence by the 2-bit symbols corresponding to the sequence ofalternating ones and zeros thereby obtaining 2-bit products in one ofthe channels; a first interpolating and pulse shaping digital filterthat filters the 2-bit products in the channel where the multiplicationis carried out; and a second interpolating and pulse shaping digitalfilter that filters the 2-bit symbols corresponding to the bits of thereal-valued baseband synchronization sequence in the other channel. 20.An apparatus for synchronizing a demodulator of alternating quadraturesdifferential binary phase shift keying signals, comprising: a circuitthat forms in-phase and quadrature components of a receivedsynchronization sequence; a circuit that convolutes the in-phasecomponent of a received synchronization sequence with impulse responsesof first and second real-valued filters, the first filter being matchedto a first sequence comprising selected in the modulator halves ofsymbols corresponding to a first pseudorandom sequence and spacesbetween the adjacent in time halves, and the second filter being matchedto a second sequence comprising selected in the modulator halves of thesymbols corresponding to a second pseudorandom sequence and spacesbetween the adjacent in time halves; a circuit that convolutes thequadrature component of the received synchronization sequence withimpulse responses of third and fourth real-valued filters, the thirdfilter being matched to a sequence comprising selected in the modulatorhalves of symbols corresponding to a first pseudorandom sequence andspaces between the adjacent in time halves, and the fourth filter beingmatched to a sequence comprising selected in the modulator halves ofsymbols corresponding to a second pseudorandom sequence and spacesbetween the adjacent in time halves; a circuit that forms an in-phasecomponent of an output signal of the complex-valued matched filtercomprising the four real-valued matched filters by summing an outputsignal of the first real-valued filter connected to the in-phase channeland an output signal of the fourth real-valued filter connected to thequadrature channel; a circuit that forms a quadrature component of theoutput signal of the complex-valued matched filter comprising the fourreal-valued matched filters by subtracting an output signal of thesecond real-valued filter connected to the in-phase channel from anoutput signal of the third real-valued filter connected to thequadrature channel; a circuit that forms a real-valued output signal ofthe complex-valued matched filter by separately squaring its outputin-phase and quadrature components and summing the squared components;and a circuit that determines the correct position of clock pulses byselecting a peak of the real-valued output signal of the complex-valuedmatched filter.